1. Field of the Invention
The present invention relates to an exhaust gas purifying system for an automobile engine.
2. Description of Related Art
Typically, some of automobile engine exhaust gas purification systems are provided with a catalytic converter for purifying nitrogen oxides in an exhaust system, in which the catalytic converter can remove nitrogen oxides (NOx) as well as hydrocarbons (HC) and carbon monoxide (CO) even when the engine is running with a lean air-fuel ratio. Such an exhaust gas purification system is known from, for instance, Japanese Unexamined Patent Publication No. 3-225013.
However, the inventors of this invention found that an exhaust gas purification system of this kind shows a phenomenon that the temperature range of catalyst activation shifts toward a lower side with passage of time under lean actual operating conditions, i.e. when the engine is running with a lean air-fuel ratio. Hence, if this phenomenon is ignored, the performance of the catalyst drops with passage of time because the air-fuel ratio is set to optimize the temperature of exhaust gas at the entrance of the catalytic converter in consideration of the initial temperature range of catalyst activation.